1. FIELD OF THE INVENTION
This invention relates to the melt-spinning of synthetic polymeric material and, more particularly, to apparatus for melting solid particles of synthetic polymer prior to extrusion.
2. DESCRIPTION OF THE PRIOR ART
Melt-spinning of synthetic polymeric material typically involves supplying the polymer in particulate or solid form to a melting device, from which the polymer is withdrawn in the molten state and forwarded, e.g., by a metering pump, to a spin pot assembly which includes a spinnerette through which molten polymer is extruded. Suitable melting devices are ordinarily either of the screw extrusion or heated element type, although other devices are known. The present invention is directed toward a melt vessel which contains heating elements.
The prior art is replete with various types of heating elements. The grid is a well known heating element and many alternative designs have been developed, e.g., perforated plates; ribs disposed in parallel vertically or horizontally; and flat spiral coils. The popular alternate to the use of the heated grid is the use of hollow heated members or fins which are disposed within the cavity of a melt vessel. These grids or hollow members are suitably heated so that polymer chip coming into contact therewith melts, to fall into a melt pool which has accumulated therebelow. It is also known to heat the wall or a portion thereof which forms the melt vessel.
In order to achieve increased production in the melt-spinning of synthetic polymers, the melting capacity of the melt vessel must be increased. Unfortunately, an increase in the cross-sectional area, and thus melting capacity, of the grid-type heating elements of the prior art necessitates a larger melt vessel. A larger melt vessel in turn increases both the risk of subjecting the polymer to differential heat profiles in the melt vessel and the risk of molten polymer degradation due to increased residence time. Therefore, it is desirable to achieve an expanded melting surface while restricting to a minimum expansion of the melt vessel itself.
It is also desirable to keep the rheological properties of the polymer at its point of withdrawal from the vessel as uniform as possible. To this end, the residence time of the molten polymer in the melt vessel should be as short as possible, and the temperature of all polymer contact portions of the melt vessel, including heating elements, should be as uniform as possible.
The melting apparatus of the present invention achieves all of these objectives, namely: increased melting capacity, more uniform melt temperatures, and improved polymer flow.